42 unhealthy days in the top county, California dominates

San Bernardino County, California, leads the 2025 ranking with 42 total unhealthy days (38 Unhealthy + 4 Very Unhealthy + 0 Hazardous), a maximum AQI reading of 207 and median AQI of 90. Of the top 10 counties, 3 are in California: San Bernardino, Riverside, Los Angeles. The South Coast Air Basin (Riverside + San Bernardino + Los Angeles) historically sits in EPA's "extreme" ozone-nonattainment classification, and these rankings reflect that pattern, three California counties consistently report the highest unhealthy-day burden in the country.

Wildfire-driven peaks, max AQI versus median AQI

A key signal in this dataset is the gap between median and maximum AQI. 8 of the top 10 counties recorded at least one day with AQI ≥ 200 (Very Unhealthy or worse), while most have median AQI under 90. This is the wildfire signature: a small number of extreme PM2.5 days driving the unhealthy-day count, while most days remain in the Good or Moderate range. El Paso County, Texas (max AQI 2122); Dona Ana County, New Mexico (max AQI 528) exceeded EPA's standard AQI scale (which caps at 500), indicating extreme post-wildfire PM2.5 events.

Comparison to the national baseline

Across the 896 counties with sufficient 2025 monitoring data, the national average median AQI is approximately 39.6 with a national average good-day percentage of 75.5%. The top-10 counties in this ranking show good-day percentages between 8.8% (Harris) and 87.7% (Platte), well below the national mean. EPA AQS, Annual AQI Summary, 2025

Why two-thirds of counties are missing

The dataset covers approximately 896 counties with active monitors. EPA's methodology guidance notes that approximately two-thirds of U.S. counties have no permanent air-quality monitor, so any "Top 10" ranking is necessarily a ranking among monitored counties, not the country as a whole. Rural Western and Mountain West counties may have similar wildfire-driven peaks that go undocumented. See the methodology page for full monitor-coverage caveats.

State Implementation Plans (SIP) and federal preemption

Under the Clean Air Act (CAA), each state submits a State Implementation Plan to the EPA documenting how it will achieve and maintain the National Ambient Air Quality Standards (NAAQS) for the six criteria pollutants. When EPA determines a SIP is inadequate, the agency may impose a Federal Implementation Plan under CAA section 110(c). California operates the country's only state-level mobile-source program, granted authority under CAA section 209 waivers issued by EPA, over 100 waivers have been issued since 1968 covering tailpipe standards, zero-emission vehicle mandates, and Low-NOx fleet rules. Other states adopt California standards under section 177 piggyback authority.

Nonattainment area classifications (marginal, moderate, serious, severe, extreme for ozone; moderate or serious for PM2.5 and PM10) trigger progressively stricter permitting, offset ratios, and Reasonable Available Control Technology (RACT) requirements. The South Coast Air Basin (Los Angeles), San Joaquin Valley, and Houston-Galveston-Brazoria are designated extreme or severe ozone nonattainment under the 2015 8-hour standard. These regulatory designations align with the top-10 ranking above: 5 of the top 10 unhealthy-day counties sit inside extreme or serious nonattainment areas.

Cross-state pollution and the Good Neighbor framework

The Cross-State Air Pollution Rule (CSAPR), promulgated 2011 and revised through CSAPR Update Rule (2016), Revised CSAPR Update (2021), and Good Neighbor Plan (2023), addresses upwind state contributions to downwind nonattainment under CAA section 110(a)(2)(D)(i)(I). EPA quantifies each upwind state's significant contribution to ozone or PM2.5 violations in downwind states using photochemical air-quality modeling (CAMx, CMAQ) and sets emissions budgets for power plants and industrial sources. The 2023 Good Neighbor Plan also covers non-EGU sources for the first time.

Monitoring infrastructure and measurement methods

The Air Quality System (AQS) is the federal repository administered by EPA Office of Air Quality Planning and Standards (OAQPS) at Research Triangle Park, North Carolina. Submissions arrive from approximately 4,000 monitors operated by state, local, and tribal (SLT) air agencies under 40 CFR Part 58. Each criteria pollutant has prescribed Federal Reference Methods (FRM) and Federal Equivalent Methods (FEM) per 40 CFR Part 53. PM2.5 sampling uses gravimetric FRM samplers for 24-hour integrated mass measurement; continuous PM2.5 monitoring uses Tapered Element Oscillating Microbalance (TEOM), Beta Attenuation Monitors (BAM), or Synchronized Hybrid Ambient Real-time Particulate (SHARP) instruments with FEM designation.

Health-effects evidence base

EPA Integrated Science Assessments (ISA) for criteria pollutants synthesize epidemiological and toxicological evidence supporting NAAQS revision. The 2019 Particulate Matter ISA documented causal relationships between long-term PM2.5 exposure and cardiovascular mortality, all-cause mortality, lung cancer, and respiratory effects. Long-term cohort studies (American Cancer Society Cancer Prevention Study II, Harvard Six Cities Study, Medicare Cohort) provide concentration-response functions used in EPA Benefits Mapping and Analysis Program (BenMAP) cost-benefit analyses. The Krewski et al. (2009) reanalysis of ACS CPS-II established the canonical 6-percent all-cause mortality risk per 10-µg/m³ PM2.5 increment.

For full methodological documentation including data lineage, monitor-network assumptions, and AQI computation steps, see the methodology page.